1. Field of the Invention
The present invention relates generally to a natural gas liquefaction system that employs a non-volatile refrigerant in one or more of its main refrigeration cycles. In another aspect, the invention concerns a cascade-type natural gas liquefaction system that employs carbon dioxide as the primary refrigerant in at least one of its main refrigeration cycles.
2. Description of the Prior Art
It is common practice to cryogenically liquefy natural gas for transport and storage. The primary reason for the liquefaction of natural gas is that liquefaction results in a volume reduction of about 1/600, thereby making it possible to store and transport the liquefied gas in containers of more economical and practical design. For example, when gas is transported by pipeline from the source of supply to a distant market, it is desirable to operate the pipeline under a substantially constant and high load factor. Often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline. In order to shave off the peaks where demand exceeds supply, it is desirable to store the excess gas in such a manner that it can be delivered when the supply exceeds demand, thereby enabling future peaks in demand to be met with material from storage. One practical means for doing this is to convert the gas to a liquefied state for storage and to then vaporize the liquid as demand requires.
Liquefaction of natural gas is of even greater importance in making possible the transport of gas from a supply source to market when the source and market are separated by great distances and a pipeline is not available or is not practical. This is particularly true where transport must be made by ocean-going vessels. Ship transportation in the gaseous state is generally not practical because appreciable pressurization is required to significantly reduce the specific volume of the gas which in turn requires the use of more expensive storage containers.
In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to xe2x88x92240xc2x0 F. to xe2x88x92260xc2x0 F. where it possesses a near-atmospheric vapor pressure. Numerous systems exist in the prior art for the liquefaction of natural gas by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the liquefaction temperature is reached. Cooling is generally accomplished by heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, and methane or a combination of one or more of the preceding. In the art, the refrigerants are frequently arranged in a cascaded manner and each refrigerant is employed in a closed refrigeration cycle. Further cooling of the liquid is possible by expanding the liquefied natural gas to atmospheric pressure in one or more expansion stages. In each stage, the liquefied gas is flashed to a lower pressure thereby producing a two-phase gas-liquid mixture at a significantly lower temperature. The liquid is recovered and may again be flashed. In this manner, the liquefied gas is further cooled to a storage or transport temperature suitable for liquefied gas storage at near-atmospheric pressure. In this expansion to near-atmospheric pressure, some additional volumes of liquefied gas are flashed. The flashed vapors from the expansion stages are generally collected and recycled for liquefaction or utilized as fuel gas for power generation.
One disadvantage of conventional LNG production facilities is their use of volatile hydrocarbon-based refrigerants to cool the natural gas. The use of such volatile hydrocarbon-based refrigerants necessitates the presence of expensive safety equipment to guard against catastrophe in the event of refrigerant leakage and/or ignition. The use of volatile hydrocarbon-base refrigerants can be especially disadvantageous when the LNG facility is located offshore. Offshore LNG plants employing volatile hydrocarbon-based refrigerants must take extra precautions to ensure that there is no leakage of the hydrocarbon-based refrigerants, which could necessitate dangerous and expensive cleanup actions.
As with all hydrocarbon production and processing facilities, capital expense and operating expense are key factors in determining the economic feasibility of a LNG plant. Thus, design engineers are always looking for ways to decrease capital expense by eliminating unnecessary equipment. Further, design engineers are constantly search for ways to reduce operating expense by making the plant run more efficiently.
It is, therefore, an object of the present invention to provide a LNG facility having a reduced amount of volatile refrigerants employed therein.
Another object of the invention is to provide a natural gas liquefaction system having enhanced efficiency, thereby reducing operating expense.
Yet another object of the invention is to provide a natural gas liquefaction system having a reduced number of vessels and equipment, thereby reducing capital expense.
It should be noted that the above-listed objects of the invention need not all be accomplished by the invention claimed herein. In addition, other objects and advantages of the present invention will be readily recognized by one skilled in the art in view of the following detailed description of the preferred embodiments, drawing figures, and claims.
In one embodiment of the present invention, there is provided a process for liquefying natural gas comprising the steps of: (a) cooling a natural gas stream in a first refrigeration cycle employing a first refrigerant comprising predominately carbon dioxide; and (b) downstream of the first refrigeration cycle, further cooling the natural gas stream in a second refrigeration cycle employing a second refrigerant comprising predominately methane.
In another embodiment of the invention, there is provided a process for liquefying natural gas comprising the steps of: (a) cooling a natural gas stream in a carbon dioxide refrigeration cycle employing a plurality of separate chillers for sequentially transferring heat from the natural gas stream to a carbon dioxide refrigerant comprising predominately carbon dioxide, said carbon dioxide refrigeration cycle including a carbon dioxide compressor for increasing the pressure of the carbon dioxide refrigerant to a discharge pressure of at least about 900 psia; and (b) downstream of the carbon dioxide refrigeration cycle, further cooling the natural gas stream in a methane refrigeration cycle employing a methane refrigerant comprising predominately methane.
In still another embodiment of the invention, there is provided a process for liquefying natural gas comprising the steps of: (a) cooling a natural gas stream in a carbon dioxide refrigeration cycle employing a plurality of separate chillers for sequentially transferring heat from the natural stream to a carbon dioxide refrigerant comprising predominately carbon dioxide, said carbon dioxide refrigeration cycle including a carbon dioxide compressor for increasing the pressure of the carbon dioxide refrigerant to a discharge pressure of at least about 800 psia; (b) downstream of the carbon dioxide refrigeration cycle, further cooling the natural gas stream in an ethylene refrigeration cycle employing an ethylene refrigerant comprising predominately ethylene; and (c) downstream of the ethylene refrigeration cycle, further cooling the natural gas stream in a methane refrigeration cycle employing a methane refrigerant comprising predominately methane.
In a further embodiment of the present invention, there is provided a LNG plant for liquefying a natural gas stream. The LNG plant comprises a carbon dioxide refrigeration cycle and a methane refrigeration cycle. The carbon dioxide refrigeration cycle comprises a carbon dioxide compressor, a carbon dioxide chiller, and a carbon dioxide refrigerant comprising predominately carbon dioxide. The carbon dioxide compressor is operable to increase the pressure of the carbon dioxide refrigerant. The carbon dioxide chiller is operable to transfer heat from the natural gas stream to the carbon dioxide refrigerant. The methane refrigeration cycle comprises a methane compressor, a methane chiller, and a methane refrigerant comprising predominately methane. The methane compressor is operable to increase the pressure of the methane refrigerant. The methane chiller is operable to transfer heat from the natural gas stream to the methane refrigerant.